The present invention is directed to a mobile cellular radio telephone system having digital voice/traffic capacity wherein digital control channels may occupy the same radio channel time slots as voice/traffic channels. More specifically, the present invention relates to a method and system for performing the access function on a digital control channel.
The first cellular mobile radio systems in public use were generally analog systems for the transmission of speech or other analog information. The systems comprised a plurality of radio channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had comparatively large coverage cells. More recently, digital cellular mobile radio systems for public use have been designed.
Digital cellular mobile radio systems comprise digital channels for transmitting digital or digitized analog information between base and mobile stations, by transmitting digitally modulated radio signals. Digital cellular mobile radio systems offer substantial advantages over analog cellular mobile radio systems.
One digital mobile radio system intended to be used as a common system for many European countries is the GSM system. In European countries already having an analog cellular mobile system, the new digital GSM system is intended to be introduced as a new system which is independent of any existing analog system. The GSM system base and mobile stations have not been designed to be compatible with existing systems; rather, they are designed to give optimum performance in various aspects within the system and of the system itself. Accordingly there has been a comparatively great freedom of choice in technical matters in the design of the GSM system.
Rather than introduce a new independent digital cellular mobile radio system, like the GSM system, in an area with an existing analog cellular system, it has been proposed to introduce a digital cellular mobile radio system which is designed for cooperation with the existing analog cellular mobile radio system. In order to obtain digital channels within the frequency band allotted to cellular mobile radio systems, there have been proposals to withdraw a number of radio channels allotted to the present analog mobile radio systems and use them in the digital cellular mobile radio system. Due to the proposed design of the digital mobile radio system, three or possibly six digital channels may occupy the same frequency band of one previous analog radio channel by using time division multiplexing. Accordingly, replacing some analog channels by digital channels in time division multiplex may increase the total number of channels.
The intended result is to gradually introduce the digital system and to increase the number of digital traffic channels while decreasing the number of analog traffic channels in the coexisting cellular systems. Analog mobile stations already in use will then be able to continue to use the remaining analog traffic channels. Meanwhile, new digital mobile stations will be able to use the new digital traffic channels. Dual-mode mobile stations will be able to use both the remaining analog and the new digital traffic channels.
With the addition of the new digital traffic channels, a corresponding need for new digital control channels arises. The conventional dual-mode systems for the most part utilize existing analog channels, such as dedicated frequencies, as the control channel.
However, the new fully digital systems will use digital control channels which occupy TDMA time slots of the same type as used for digital traffic channels.
Such digital control channels have earlier been used in the European GSM system. In GSM an access is always performed using a single burst in order to minimize occupancy of the channel dedicated for accesses. Any subsequent actions like authentication and ciphering are performed on another type of control channel assigned after the actual access. This use in GSM of several types of control channels implies considerable complexity. For this reason, and for better compatibility with the existing North-American dual mode system, it is preferred to use only one type of uplink control channel for the fully digital system in the USA. A consequence of this decision is that the control channel used for access in the USA must be able to handle other functions besides access such as authentication or ciphering, which require more than one burst in a message. The present invention solves the special problems associated with the use of multi-burst access messages. The present invention further solves the problem of access burst collisions in very large cells.
It is an object of the present invention to provide a procedure usable by a mobile station to access the land system via a digital control channel, taking into consideration that existing EIA/TIA-54 formats for digital traffic channels shall be used with minimum modification for the digital control channel in order to achieve low hardware complexity in the mobile station. This is accomplished by using the same format for both the traffic channels and the control channels except that the CDVCC and SACCH fields of the traffic channel are used on the control channel to perform the function of structuring the access to the land system from a mobile.
It is a further object of the invention to avoid access burst collision in very large cells. This is accomplished by defining two separate access methods, one to be used in very large cells and the other to be used in smaller cells. According to an access method to be used in very large cells, a mobile station transmits two access bursts, a first shortened access burst containing mainly synchronization information and enabling time-alignment information for the mobile station to be derived and a second normal-length access burst containing the usual access information.
It is a further object of the invention to provide sufficient synchronization information in an access burst to ensure good reception. This is achieved by using the CDVCC and SACCH fields defined in existing EIA/TIA-54 formats to transmit additional synchronization information.
It is a further object of the invention to provide an access method with a low probability of causing multiple accesses, i.e. an access method not making more than one base station believe that it has been granted access. This is achieved by using the digital verification color code DVCC when coding/decoding the cyclic redundancy check CRC the same way as for the traffic channels. In other words, the xe2x80x9cimplicitxe2x80x9d DVCC is still used whereas the xe2x80x9cexplicitxe2x80x9d DVCC (the CDVCC field in the message) is used for the access procedure.
The two fields CDVCC and SACCH have 12 bits each for a total of 24 bits that may be used on the digital access channel. Besides being used to improve synchronization, especially in the uplink direction, the 24 bits may further be used to distinguish between control channels and traffic channels and to carry reservation information for multi-burst accesses. For example, the 24 bits may carry one out of two possible patterns uplink and one out of two possible patterns downlink. Thereby two things are accomplished: firstly, in addition to improving synchronization uplink, distinction of a control channel from a traffic channel is facilitated downlink by using, in addition to the standardized 28-bit sync word, two of the possible 24-bit patterns in the CDVCC-SACCH fields as distinguishing sync words (the other (2**24)-2 patterns are illegal); secondly, transmitting downlink the messages xe2x80x9cfreexe2x80x9d or xe2x80x9creservedxe2x80x9d and uplink the messages xe2x80x9creservexe2x80x9d or xe2x80x9cdo not reservexe2x80x9d as one out of the two allowed 24-bit patterns virtually excludes misinterpretation.